Controlled Synthesis of Enantioselective 1-Aminoindenes via Cobalt

Research Article Cite This: ACS Catal. 2018, 8, 9364−9369

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Controlled Synthesis of Enantioselective 1‑Aminoindenes via Cobalt-Catalyzed [3 + 2] Annulation Reaction Min-Hsien Chen,† Jen-Chieh Hsieh,*,‡ Yi-Hsien Lee,‡ and Chien-Hong Cheng*,† †

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan Department of Chemistry, Tamkang University, New Taipei City 25137, Taiwan

‡

ACS Catal. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/17/18. For personal use only.

S Supporting Information *

ABSTRACT: A cobalt-catalyzed synthesis of 1-aminoindenes via enantioselective [3 + 2] annulation is described. In this reaction, the desired products can be selectively prepared in either (R)- or (S)-form by the ligand-controlled synthesis, which is initiated by the cleavage of C−B, C−Br, or C−O bonds under very mild reaction conditions. In addition, this enantioselective cobalt catalysis provides high regioselectivity for both internal and terminal alkynes, and affords products generally in high yields and ee values. KEYWORDS: enantioselective synthesis, [3 + 2] annulation, 1-aminoindene, cobalt, chiral PHOX ligand

A

minoindene and its analogues are important fragments; they are frequently found in natural alkaloids1 and known as an important structural motif in many pharmaceutical products. For example, the traditional herbs used as antipyretic,2 the depression medicines,3 the potential drugs of atrial fibrillation,4 and antidiabetic5 have been reported to contain aminoindene core structures. Apart from the pharmaceutical compounds, aminoindenes are also applied in material chemistry6 and used as ligands for metal complexes.7 These applications attract attention from many organic chemists and result in continuous exploration of new synthetic strategy. The transition-metal-catalyzed [3 + 2] annulation reaction is a facile and often selected method to synthesize aminoindenes and their derivatives. Most of the methods involving this concept were carried out by the cyclization of o-functionalized arylimines with alkynes or alkenes. In this respect, a variety of catalysts based on Re,8 Rh,9 Pd,10 Ru,11 Co,12 and Ir13 have been reported for this transformation. However, the enantioselective version of the [3 + 2] annulation reaction is quite limited. Up to now, only a few studies have described the enantioselective synthesis of aminoindenes (Scheme 1). In 2011, Cramer first reported the catalytically enantioselective [3 + 2] annulation to afford chiral 1-aminoindenes by using rhodium complex with additional bidentate chiral phosphine ligand as the catalyst,9d which is specially applied to create quaternary chiral centers, and has been further extended to synthesize spiro-compounds in very recent years.9f The second enantioselective catalysis for the synthesis of chiral 1-aminoindenes was also reported in 2011 by Yu and Lu,10b which involves the cyclization of (S)-2-(N-tert-butanesulfinylimino)arylboronic acids with alkynes by using preprepared chiral cationic palladium complex as the catalyst. In this reaction, the N-attached (S)-tert-butylsulfinyl group is necessary to facilitate its enantioselectivity. And to determine the ee values, further oxidation to © XXXX American Chemical Society

Scheme 1. Enantioselective Synthesis of 1-Aminoindenes

convert the sulfinyl group to the corresponding sulfonyl group is required, which restricts the resulted products possessing a tert-butylsulfonyl group on the nitrogen. Even though the pioneer works had made a contribution in the enantioselective synthesis of 1-aminoindene, the progress of its synthetic methods is still demanded in response to the requirement of structural diversity. Our continuous interest in the late-transition-metal catalyzed annulation reactions by utilizing the C−N multiple bonds as the reaction participants12b,14 Received: July 4, 2018 Revised: August 9, 2018 Published: August 27, 2018 9364

DOI: 10.1021/acscatal.8b02566 ACS Catal. 2018, 8, 9364−9369

Research Article

ACS Catalysis Table 1. Optimization of Reaction Conditionsa

entry

[Co]/ligand

additive (x)

base

yield (%)b

ee (%)c

1 2 3 4 5 6 7 8 9 10e 11f 12e 13e 14e 15e

CoCl2/L3 CoCl2/L4 CoCl2/L5 CoCl2/L6 CoCl2/L7 CoCl2/L8 CoCl2/(R)-Ph-PHOX CoCl2/(R)-Ph-PHOX CoCl2/(R)-Ph-PHOX CoCl2/(R)-Ph-PHOX CoCl2/(R)-Ph-PHOX Co[(R)-Ph-PHOX]Cl2 Co[(R)-Ph-PHOX]Cl2 Co[(R)-Ph-PHOX]Cl2 Co[(R)-Ph-PHOX]Cl2

none none none none none none none none none none none none ZnI2 (20) ZnCl2 (20) ZnCl2 (40)

NaHCO3 NaHCO3 NaHCO3 NaHCO3 NaHCO3 NaHCO3 Na2CO3 K2CO3 K3PO4 NaHCO3 NaHCO3 NaHCO3 NaHCO3 NaHCO3 NaHCO3

66 73 77 52 (66)d 90 87 54 76 0 33 17 45 67 82 89

14 50 51 85 77 3 80 83 90 90 91 91 91 84

a

Conditions: 1a (0.2 mmol), 2a (1.5 equiv), cobalt complex/ligand (10 mol %, 1:1), additive (20−40 mol %), base (1.0 equiv), CH3CN (1 mL). Isolated yield. cDetermined by chiral HPLC analysis. dNMR yield (mesitylene as the internal standard). e60 °C. f40 °C.

b

for various arylboronic acid 1 and alkyne 2 was then examined (Scheme 2). In general, the reactions worked well for selected N-attached aryl groups. Slightly reduced performances were found in yield for the para-anisyl group (S-3b), and in ee value for the difluorophenyl group (S-3f). The product S-3c has been verified as an (S)-isomer by single-crystal X-ray diffraction analysis. All compounds revealed in Scheme 2 have similar trends in their optical rotation and retention time. Substituents on the alkyne are influential in both of the yields and the ee values. The reactions for electron-deficient alkynes slightly depressed the yields and the ee values (S-3g and S-3h), while the reaction for electron-rich alkyne remained high yield and ee value (S-3i). Alkyne with the bilateral methoxymethyl groups can be tolerated in this cobalt catalytic reaction as well. However, the corresponding product 3j was provided without any enantioselectivity. The unexpected coordination of oxygen to catalyst might obstruct the function of chiral PHOX ligand. Substituent on the moiety of arylboronic acid does not have significant affect on the yield and the enantioselectivity. Both electron-donating groups and electron-withdrawing groups can be tolerated to afford the desired products in moderate to good yields with high ee values (S-3k to S-3n). Notably, the enantioselective cobalt catalysis also demonstrates excellent stereoselectivity in the step of 1,2-insertion of alkyne. When unsymmetrical alkynes were introduced to the reaction, the corresponding products (S-3o and S-3p) were afforded in single stereoisomer with high yields and good ee values (see the NOE data of product S-3p in Supporting Information). The present Co-catalyzed enantioselective [3 + 2] annulation can also proceed smoothly by replacing arylboronic acids 1 with aryl bromides 4 (Scheme 3). With fine-tuning of reaction conditions, the (R)-isomers were provided in good

prompted us to develop an asymmetric annulation for chiral 1-aminoindenes through the addition of C−N multiple bond. Herein, we report a controlled synthesis of the cobalt-catalyzed [3 + 2] enantioselective annulation of o-imidoylarylboronic acids (and arylhalides, pseudohalide) with alkynes to afford chiral 1-aminoindenes. Our initial study started from the survey of chiral ligands (Table 1, entries 1−6; see Table S1 in Supporting Information for the detail of optimization study). By screening a series of chiral ligands, we found P,N-type bidentate chiral ligands are more effective to proceed this enantioselective catalysis than P,P-type and N,N-type bidentate ligands, and (R)-Ph-PHOX (L6) provided the highest efficacy in both the reaction yield and the ee value. Further optimizations of conditions through investigating the effect of bases, additives, and temperatures were also undertaken and summarized below (entries 7−15). Carbonate base, especially the K2CO3, provided much better yield without obvious loss in its ee value (entry 8). However, we were not able to have any improvement in the ee values by introducing the K2CO3 as base in further tuning of the reaction conditions. Decreasing the temperature is helpful for the enantioselectivity but pernicious for the yields (entries 10 and 11). Thus, we reduced the reaction temperature from 80 to 60 °C for other examinations. We further found that an integrated catalyst can slightly increase both yield and ee value (entry 12), and the Lewis acid is very crucial in this reaction (entries 13−15). Among the Lewis acids that we have surveyed, the zinc halides were found to have the highest improvement on the yields, and a catalytic amount of ZnCl2 dramatically enhanced the yield without loss of the enantioselectivity (entry 14). After the optimized condition of this Co-catalyzed enantioselective [3 + 2] annulation was obtained, the scope of reaction 9365

DOI: 10.1021/acscatal.8b02566 ACS Catal. 2018, 8, 9364−9369

Research Article

ACS Catalysis Scheme 2. Reaction Scope of 1 with 2a

Scheme 3. Reaction Scope of 4 with 2a

a

Condition: 1 (0.2 mmol), 2 (0.3 mmol), Co[(R)-Ph-PHOX]Cl2 (0.02 mmol), ZnCl2 (0.04 mmol), NaHCO3 (0.2 mmol), and CH3CN (1 mL) at 60 °C for 12 h. The yields refer to isolated yield. The ee values were determined by chiral HPLC analysis.

yields with high ee values by using Co[(S)-iPr-PHOX]Cl2 as the catalyst with zinc as the reducing agent to activate the cobalt species (see Table S2 for the optimization study and Scheme 5 for the proposed mechanism). For contradistinction, we confirmed the product R-3c for its absolute configuration by single-crystal X-ray diffraction analysis. Including R-3c, all enantiomers illustrated in Scheme 3 (R-3a to R-3p) and Scheme 2 possess opposite trend in their optical rotation and retention time. Not just the enantiomers of Scheme 2, but we also exhibit some additional structures (R-3A to R-3H) to evaluate the reaction scope. Compared to the reactions in Scheme 2, the reactions for aryl bromides 4 require lower temperature and additional H2O as the proton source (see Table S3).15 Generally, products R-3a to R-3p were afforded in similar yields and ee values with their enantiomers in Scheme 2 except R-3c, 3j, R-3n, and R-3p. Products R-3c, R-3n, and R-3p were provided in similar yields with their enantiomers, but in significantly lower ee values. A big difference for 3j in this protocol was shown in that 3j is not even able to be obtained from this reaction. The unsymmetrical products R-3o and R-3p were furnished in single stereoisomer as well. R-3A to R-3H were generated in 80 to 98% yields with 78 to 98% ee. Product 3I with an N-attached tosyl group was collected as a racemic mixture in 86% yield. Compared to the reactions in Scheme 2, reactions by using the aryl bromides 4 as substrates are more sensitive in their structural subunit for the enantioselectivity. Every slight change of the substituent obviously affects the ee values (R-3B vs R-3o, R-3C vs R-3k, and R-3E to R-3G). An additional unsymmetrical product R-3H generated from a long-chain internal alkyne was also afforded in single isomer (see NOE data). Under similar protocol, the reaction can be initiated from the cleavage of other carbon−halide or carbon−oxygen bonds

a

Condition: 4 (0.2 mmol), 2 (0.24 mmol), Co[(S)-iPr-PHOX]Cl2 (0.01 mmol), Zn (0.6 mmol), undistilled CH3CN (1 mL) at 40 °C for 12 h. The yields refer to isolated yield. The ee values were determined by chiral HPLC analysis.

as well. Herein, reactions to approach R-3a from different halosubstrates and pseudohalo-substrate 4 have been examined to understand their properties (Scheme 4). As indicated, when we Scheme 4. Tolerance of Halides and Pseudohalide

employed the iodo-substrate, R-3a could be obtained in 53% yield with 91% ee at 40 °C with complete consumption of the substrate. The low yield was caused by unclear side reactions. However, the chloro-substrate cannot be fully consumed even with longer reaction time or at higher temperature. The best performance was achieved at 60 °C to give R-3a in 47% yield with 89% ee. Higher temperature significantly caused lower enantioselectivity, while lower temperature reduced reactivity. We further tested the tosylate and found it is also able to be a leaving group; the best enantioselectivity was provided at 60 °C in 39% yield with 94% ee. According to the above results and the previous reports,16 a proposed reaction mechanism accounting for the Co-catalyzed enantioselective synthesis of N-arylinden-1-amine 3 is shown in Scheme 5. The reactions are likely initiated from generation 9366

DOI: 10.1021/acscatal.8b02566 ACS Catal. 2018, 8, 9364−9369

Research Article

ACS Catalysis Scheme 5. Proposed Mechanism

of active cobalt complexes. The initial Co(II) complex can be either reduced to a Co(I) complex by zinc powder (Scheme 3) or just converted to the corresponding Co(II) species by ZnCl2, in situ generated sodium salt, and metaboric acid (Scheme 2). The key intermediates and pathway for entire reaction start from A to B, and then convert to C. Protonation of the resulted complex C via the respective proton sources (boronic acid and H2O) provides the desired product 3 and regenerates the active cobalt complexes. For the reaction by using arylboronic acid 1 as substrate, the formation of complex A occurs through the transmetalation of substrate 1 to the active Co(II) species, whereas the formation of complex A′ is caused by the oxidative addition of aryl bromide 4 to Co(I). A subsequent 1,2-insertion of alkyne 2 transforms A (or A′) to B (or B′). The chiral PHOX ligand plays an important role in both complexes A (or A′) and B (or B′).17 The steric hindrance, which dominates the configuration of desired product, is controlled by the substituent attached to the nitrogen-adjacent carbon on the oxazole. As illustrated, the phenyl group of complex B makes R group far away from the oxazole moiety and leads the subsequent intramolecular addition of imine from the Re-face, while the isopropyl group of complex B′ leads to the Si-face insertion. In conclusion, we have developed the controlled synthesis of chiral 1-aminoindenes in either (S)- or (R)-form through the Co-catalyzed enantioselective [3 + 2] annulation reactions. This cobalt catalysis proceeded under very mild reaction conditions, which can be initiated from the cleavage of C−B, C−Br, or C−O bonds. In addition, the reaction scope has been found in wide diversity with high yields and good to excellent ee values. The regioselectivity is well performed for unsymmetrical alkynes. Further extension of the reactions with various π components is currently underway.

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Experimental procedures, characterization, spectral data, X-ray structure, and NMR spectra (PDF) X-ray data (CIF) X-ray data (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-C.H.). *E-mail: [email protected] (C.-H.C.). ORCID

Jen-Chieh Hsieh: 0000-0002-1975-7842 Chien-Hong Cheng: 0000-0003-3838-6845 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of Republic of China (MOST 105-2113-M-032-005-MY2) for financial support of this research.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b02566. 9367

DOI: 10.1021/acscatal.8b02566 ACS Catal. 2018, 8, 9364−9369

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ACS Catalysis

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